1. Field
The application is related to techniques for mapping internal structures in the body of an animal or human, and more particularly to such technique for localizing a medical instrument with respect to anatomical features or the like during surgical or other medical procedures.
2. State of the Art
Various scanning apparatus and methods are known for imaging and mapping body structures, which provide target location data for surgical and other medical procedures. One group of methods, including still photography, videography, radiological x-rays, and angiography, typically produces only a two-dimensional projection of a three-dimensional object. For purposes of this application, this first group will be termed "two-dimensional" or "2D" imaging.
A second group of methods, of which computerized tomographic (CT) scanning, positron emission tomography (PET) scans, and magnetic resonance (MRI) imaging are exemplary, provide three-dimensional (abbrev. "3D" herein) information about internal structures (i.e., structures not visible from the exterior of the patient). The three-dimensional information about the internal volume is reconstructed from multiple scans of a known thickness (generally about a millimeter) made along parallel planes displaced from each other by a known distance, usually of the order of millimeters. An example of such a reconstructed volume image is depicted in FIG. 1A, including the contours of a selected anatomical feature within the brain. In this application, methods in this second group will be referred to as "volume" scanning or imaging.
In performing resection or other surgical manipulations, it is highly desirable to correlate the location of instruments, patient anatomical features, or other elements or structures placed in the surgical field, and generally as seen by the surgeon, with the location of internal targets or features as visualized by one of the volume scanning techniques. Such a correlation process is often termed "localization".
A commercially available device for localization in neurosurgery is the Brown-Roberts-Wells (abbrev. BRW) localizer (U.S. Pat. Nos. 4,341,220, and 4,608,977). The BRW system includes a large ring-like structure which surrounds the patient's head and is fixed in place. The ring establishes a 3D coordinate system with respect to the patient's head. A separate calibration unit having an array of rod elements is fixed to the ring to surround the head during the production of volume scan and/or 2D images. The rod elements have known coordinates in the 3D coordinate system established by the ring, and produce spots in the volume scans. Other features in the volume scans can then be assigned coordinates in the 3D coordinate system established by the ring, by correlation with the known coordinates of the rod elements producing the spots.
After the images are made, the calibration unit is detached from the ring, and a guidance arc calibrated to the 3D coordinate system of the ring is attached in its place. The guidance arc provides coordinate reference information which may be used to guide a medical instrument. The medical instrument is usually attached to the guidance arc.
The BRW system has several disadvantages. The ring is cumbersome and uncomfortable for the patient, but it must be affixed in place when the volume and/or 2D scans are made, and kept there until the medical procedure is complete. It is possible to remove the ring after the scans are made, but precise repositioning is critical to avoid error in localization. Accurate repositioning is difficult, so present practice generally is to keep the ring in place until after the surgery. When not attached to the guidance arc, the position of a medical instrument in terms of the 3D coordinate system of the ring, and therefore in respect to the features identifiable in the volume or 2D scan, is not accurately known.
U.S. Pat. No. 4,618,978 to Cosman discloses a localizer device for use with a BRW-type system, including an open box composed of connected rods, which surrounds the patient's head and constitutes a calibration unit.
Alternatively, cranial implants of radio-opaque or MRI-opaque materials can be made. Generally, a minimum of three implants are required for establishing a three-dimensional space in volume scans. At present this method is considered very undesirable, in part because of the risk of infection or other complications of the implants.
Accordingly, a need remains for rapid, reliable, and inexpensive means for localizing a medical instrument relative to points of interest including both visible anatomical features and internal features imaged by volume and/or 2D methods. A need further remains for such means which does not require the physical attachment of a reference unit such as the BRW ring to the patient. Highly desirably, such means would be useful to track the position of a medical instrument in real time, and without requiring that the instrument be physically attached to a reference guide.